This invention relates to the field of magneto-optic recording. More particularly, it relates to improvements in apparatus for inverting (i.e. changing the polarity of ) a magentic field through which a magneto-optic recording element passes during the information recording and erasing steps of the magneto-optic recording process.
In the magneto-optic recording process, a vertically magnetizable recording layer is initially sensitized by simultaneously subjecting it to a uniform magnetic field and a temperature which exceeds its Curie temperature (e.g.400.degree. C.). The magnetic field, being directed perpendicular to the recording layer, serves to uniformly align all of the magnetic domains therewith. Once all the magnetic domains are facing in the same direction, the recording layer is ready to record information. Such recording is effected by subjecting the recording layer to a magnetic field of reverse polarity while scanning the layer with an intensity-modulated laser beam.
During the recording process, the laser beam intensity is switched between high and low levels, representing the digital (binary) information being recorded. Only the high level is sufficiently intense to raise the temperature of the irradiated portion of the recording layer to above its Curie temperature, thus, digital information is recorded at the point of incidence of the laser as the more intensely irradiated magnetic domains flip in orientation to align themselves with the reverse magnetic bias field. Playback of the recorded information is commonly achieved by scanning the information tracks with a plane-polarized beam of radiation and monitoring the reflected beam for shifts in the plane of polarization, as produced by the well-known Kerr effect. To erase the recorded information, the polarity of the applied external magnetic field is reversed, and the recording layer is scanned with a beam of sufficient intensity to again heat the recording layer to above its Curie temperature. After this erasure step, all of the irradiated magnetic domains will again face in the same direction.
Various schemes have been proposed to achieve the magnetic field inversions required in switching between the record and erase modes of the magneto-optic recording process. In the disclosure of U.S. Pat. No. 4,701,895, for example, a cylindrical permanent magnet is positioned radially with respect to a spinning magneto-optic disk. The poles of the magnet are diametrically opposed, and the magnet is supported for rotation about its longitudinal axis. Circular eddy currents induced by the cylindrical magnet on the surface of the spinning disk, serve to generate a second magnetic field which opposes that of the magnet. As a result, the cylindrical magnet rotates freely about its axis, driven by the eddy current-produced magnetic field on the disk. A mechanism is provided for stopping the rotation of the magnet with one or the other of its poles located adjacent the disk. Depending on the orientation of the permanent magnet, either a recording or erasing magnetic bias field is provided.
In the magnetic field inversion scheme described above, the eddy current-induced magnetic field used to rotate the permanent magnet is relatively weak. Hence, fast a reliable field reversals are not attainable. Moreover, since the amplitude of the eddy currents is dependent on the electrical conductivity of the recording layer, certain types of recording layers may not be useful in producing the required driving force for rotating the magnet.
U.S. Patent No. 4,748,606, discloses various other magnetic field-inverting mechanisms. In all such mechanisms, a multiturn coil is used to flip (by 180.degree.) the orientation of a rotatably supported bar magnet relative to a magneto-optic recording element. The coil surrounds the magnet and, in response to a current pulse being applied to the coil, produces a transient magnetic field tending to repel the field of the permanent magnet. To assure that this transient field applies a rotational force (torque) on the bar magnet, the center of the magnetic field of the bar magnet is displaced with respect to that of the magnetic field winding (coil). Alternatively, the necessary torque is achieved by arranging a ferromagnetic (e.g. iron) strip or auxillary bar magnet along one side of the rotary housing of bar magnet to cause the field of the bar magnet to be slightly inclined, relative to the field of the coil.
While the use of a separate magnetic field winding provides a more positive torque on the permanent magnet than that produced by magnetically-induced eddy currents, the mechanisms disclosed in the '606 patent are disadvantageous in that they require considerable time for the bar magnet to settle to a steady-state position after pole-flipping (i.e., 180.degree. rotation ) has occurred. Since there is no means provided for damping the inherent oscillating movement of the bar magnet about its two nominal positions, the position of the magnet tends to oscillate prior to settling to its nominal position. This oscillation, of course, adversely impacts the rate at which the magneto-optic process can be switched between recording and erase modes. While stop mechanisms, such as disclosed in the '895 patent, could be used to eliminate this oscillation problem, such mechanisms add complexity and cost to the system.